1. Field of the Invention
The present invention relates to a semiconductor integrated circuit having a circuit block which is constituted of a transistor with a low threshold voltage, and to which the supply of a power supply voltage is suspended while it is in nonoperation.
2. Description of the Related Art
In accordance with the miniaturization of the semiconductor element structure, an insulation film constituting a transistor has become thinner, which often lowers reliability of the transistor. A power supply voltage supplied to a semiconductor integrated circuit has been decreasing year by year in order to prevent reliability deterioration.
When the power supply voltage is lowered and the difference between the power supply voltage and a threshold voltage of the transistor is thereby reduced, it is difficult for the transistor to turn on, resulting in a decrease in the operating speed. In order to prevent this, the threshold voltage of the transistor has been lowered in accordance with reduction in the power supply voltage.
Moreover, the miniaturization of a transistor has been accompanied by an increase in a leak current (subthreshold current) when the transistor is in nonoperation. The increase in the subthreshold current results in increase in power consumption during a standby period of a semiconductor integrated circuit. The increase in power consumption is a serious problem for portable equipment using battery.
Japanese Unexamined Patent Application Publication Nos. Hei 5-210976 and Hei 7-212217 disclose a technique called MTCMOS (Multi-Threshold voltage CMOS) for reducing the subthreshold current. In the MTCMOS technique, the threshold voltage of a transistor in a circuit block requiring a high-speed operation is set low, and a power supply terminal of the circuit block is connected to a power supply line via a switch transistor with a high threshold voltage. Then, keeping the switch transistor on while the circuit block is in operation and off while the circuit block is in nonoperation reduces the power consumption during a standby period.